This invention is concerned with a hood for vapor coating glass containers to control the amount and placement of the metal-oxide coating on the container sidewalls and finish of the glass containers. In the past, the amount and placement of a coating, e.g., tin oxide, on the finish of a glass container has been difficult to control resulting in excessive coating on the finish of the glass container. This problem of excessive metal-oxide coating on the finish of a glass container occurs during a coating process wherein tin tetrachloride (SnCl.sub.4) vapors flow over and chemically react with the hot surfaces of the glass container.
The desirability of applying a metal-oxide coating to the exterior of a glass container has long been recognized. Such coatings, which include tin, titanium, or other reactive metallic compounds, or organometallic compounds, protect the glass container from surface damage, such as abrasions and scratches, which result in a loss of tensile strength for the glass container. The need for high tensile strength in a glass container is particularly acute when containers are mass produced, move rapidly in close proximity along high speed conveyor lines, and are subsequently filled with carbonated beverages, beer, wine, foodstuffs and the like that produce gaseous pressure within the container.
The metal-oxide coating is typically applied when the glass container emerges in a heated, fully shaped condition from a glassware forming machine, that is at the "hot end" of the system. The containers are transported away from the forming machine by a conveyor. Temperatures in excess of 400 degrees Centigrade exist at the surface of the glass containers, so that when a heat decomposable inorganic metallic, or organometallic, compound is applied thereto, the compound reacts immediately and is converted to a metal-oxide coating.
One well known and previously used technique for applying a metal-oxide coating to the hot glass containers calls for spraying the opposite sides of the containers as they travel on a conveyor, in single file, through two longitudinally spaced, oppositely positioned spray heads. Each spray head covers approximately 180 degrees of the circumference of the bottle, so that at least two spraying stations are required to coat the entire glass container. Receivers are positioned at the opposite sides of the conveyor in alignment with the spray heads. Pressurized air with the coating compound entrained therein is discharged from each spray head at a significant pressure, while the receivers are usually maintained at a relatively low pressure. The resultant pressure differential increases the velocity, and thus the effectiveness, of the coating-precursor compound. Coating systems of this nature are disclosed, inter alia, in U.S. Pat. No. 3,516,811, to Gatchet, et al., and U.S. Pat. No. 3,684,469, to Goetzer, et al.
Gatchet, et al. recognized that the deposition of a metal-oxide coating on the finish of the glass container passing on a conveyor through the open-sided coating apparatus of the prior art was undesirable, as noted in column 3, lines 21-57 of U.S. Pat. No. 3,516,811. Gatchet, et al. relied upon spray heads producing a theoretically laminar flow which would pass laterally across the width of the conveyor to control the location as well as the uniformity of the metal-oxide deposit, as shown in FIG. 4 of that patent.
The above-described coating systems, however, are what may be termed "open-sided", and are thus adversely influenced by ambient conditions in the facility where the glass containers are formed. The ambient conditions of prime concern are rapidly-moving air currents, moisture in the air, and the potentially toxic and corrosive fumes and pollutants being discharged from the spray heads. Air currents can cause turbulent conditions at the spray heads, which can in turn result in a preferential or uneven application of the protective coating. Some of the coating will therefore accumulate on the bottle "finish", the term used in the industry to designate the closure region of the bottle. The rapidly-moving air currents disrupt the laminar-flow patterns which are theoretically possible with open-sided systems, and the capability for uniformly, and consistently, applying the same thickness of coating is seriously reduced.
To compensate for air currents as described above, the systems are therefore operated at higher pressures, and with the use of greater amounts of coating compound, than would be required under quiescent conditions. The necessary result of process adjustments such as these is the use of greater amounts of coating compound than required for optimum economy.
The moisture in the hostile atmosphere described above causes hydrolysis loss, thus rendering some of the compound unfit for its intended purpose. Further, the escape of potentially toxic fumes into the atmosphere at the work place can constitute an occupational health hazard, and may also be a violation of applicable law. These fumes are also generally quite corrosive, and can attack various components of the glass factory, such as, e.g., blowers, exhaust systems, conveyors and roofs, obviously leading to increased plant-maintenance costs. Additionally, the efficiency of these open-sided systems is low, since much of the relatively expensive coating compound is wasted.
A second, well-known, and widely employed technique for applying a metal-oxide coating to hot glass containers relies upon a formed sheet-metal coating hood with spray heads and associated receivers situated therein. The hood obviates many of the problems associated with the open-ended spray systems discussed above. For example, it isolates the glass containers from ambient conditions, and furnishes a more controlled atmosphere, which enhances the coating operations. The hood includes an exhaust system which captures most of the air-entrained coating compound not adhering to the containers, thus reducing the problem of venting the system and minimizing the opportunity for the coating compound to attack building components. Also, the hood can significantly raise the coating efficiency of the systems, with attendant cost savings.
Coating hoods substantially representative of the prior art are disclosed in U.S. Pat. No. 3,819,404 to Scholes et al.; U.S. Pat. No. 3,933,457, to Scholes; and U.S. Pat. No. 4,389,234 to Lindner. The most recent patent to Lindner, et al. presents a coating hood including a tunnel for allowing containers to pass therethrough, and a vertically adjustable flat roof for accommodating containers of various sizes. At least two jet slots are located in each side wall, and at least two receivers or suction slots are aligned therewith. The jet and suction slots are interspersed opposite each other in each side wall. The coating compound is introduced through at least one feedpoint, and blowers secured to the side walls furnish an inner and an outer loop of high-velocity air, of which the inner loop contains the coating compound, to the interior of the hood. Baffles are situated in the flow path of the high-velocity air so that the jets issuing from the jet slots are well defined, and thus better suited for their intended function.
It will be appreciated from the foregoing that there is still a significant need for an improved apparatus for coating glass containers. Accordingly, one aspect of the present invention is to provide an apparatus that is capable of applying a consistent coating thickness about the circumference and height of a glass container. Another aspect of the present invention is to provide an apparatus for vapor coating glass containers to control the amount and placement of a metal-oxide coating on the sidewalls and the finish of a glass container. Another aspect of the present invention is to provide an apparatus for applying a metal-oxide coating to hot, freshly formed glass containers that may operate under a wide range of process conditions on a variety of different types and sizes of glass containers. Yet another aspect of the present invention is to provide an apparatus for applying a metal-oxide coating to glass containers that is simple and economical to operate.